Introduction to CMOS OP-AMPs and Comparators

CHAPTER 3

BASIC ANALOG CMOS SUBCIRCUITS

In this chapter some of the basic subcircuits commonly utilized in analog MOS integrated circuits are examined. These blocks include a variety of bias circuits, current mirrors, single-stage amplifiers, source followers, and differential stages. These subcircuits are typically combined to synthesize a more complex circuit function. The operational amplifier and comparator, covered in later chapters, are examples of how simple subcircuits are combined to form more complex functions.

The first part of this chapter covers the subject of the bias circuits in CMOS technology and the current mirrors. Next, the CMOS gain stage is introduced, with particular emphasis on the use of active devices as active loads. The current mirror subcircuit covered as a biasing element is utilized as a dynamic load to obtain very high voltage gains from a single-stage amplifier. The differential amplifier, which represents a broad class of circuits, is discussed next. The differential amplifier is one of the most widely used gain stages, whose basic function is to amplify the difference between two input signals. Finally, the last part of the chapter deals with the small-signal analysis and frequency response of CMOS amplifier stages. A good understanding of the topics presented in this chapter is essential for the analog CMOS designer, as most designs start at the subcircuit level and progress upward to realize a more complex function.

3.1. BIAS CIRCUITS IN MOS ...